The Renewable Fuel Standards (RFS) mandate will require higher volumes of advanced biofuels in the near future. This increasing need for biofuels means that future biofuel stocks will need to be fungible at high concentrations with current transportation fuels. Established biofuels, such as ethanol and biodiesel, present serious performance and stability problems due to the presence of oxygen, instability, corrosiveness and low energy equivalents.
Ethanol is the best known and most used biofuel around the world. Ethanol is directly used with gasoline at up to 10 v/v % blends in the US. Higher than 15% blends may cause unacceptable corrosion in both blending equipment and consumer cars that are not specifically equipped to deal with this biofuel. Biodiesel is also a well known biofuel used as a diesel substitute. Some states in US already require biodiesel/diesel blends of up to 2% biodiesel. Biodiesel, however, can cause engine fouling problems when used during the winter with low temperatures due to unfavorable cold flow properties. Biodiesel can also present storage and stability problems. Due to the unsaturated nature of the hydrocarbon moieties in biodiesel and the unstable oxidative bonds, fatty esters undergo hydrolysis reactions increasing the acidity of the fuel and, hence, its corrosiveness. In addition bacterial growth can take place on biodiesel during long storage periods. Finally, biodiesel viability is constrained by the current cost and availability of vegetable oils and animal fats used for its preparation. Because ethanol and biodiesel are not fungible in the current fuel system, have low energy equivalents, and corrosiveness/instability problems, other renewable fuel options need to be investigated.
Several researchers have proposed different avenues for biofuel synthesis from oxygenate precursors. Recently, Dumesic and co-workers disclosed the use of gamma-valerolactone (GVL) for biofuel synthesis (Bond, et al., 2010). Their proposed strategy includes GVL decarboxylation to butenes using a SiO2—Al2O3 catalyst at 375-400° C. and 529 psi (WHSV=0.09-0.9 h−1). The product gas stream containing butenes is passed through a separator to remove water and then is sent to a second reactor containing either ZSM-5 or Amberlyst-70 for oligomerization to C8+ olefins. GVL conversions were reported between 70-99% with butene yields in the 35-96% range. In a second reaction, butane oligomerization conversions varied from 0-93%. Dumesic and co-workers have recently also proposed the use of levulinic acid for the synthesis of C9 linear hydrocarbons (Serrano-Ruiz, 2010). They state that through isomerization C9 paraffins should make good gasoline components. They propose using a linear C9 paraffin as a diesel component given its good cetane number. Unfortunately some C9 paraffins may be too light for diesel blending. Another strategy for diesel fuel synthesis is the oligomerization of C9 olefins obtained from the dehydration of 5-nonanol. The 5-nonanol may be obtained directly from the hydrogenation of 5-nonanone that is the ketonization product of valeric acid (VA). Recently, Huber and co-workers propose a four-step process for the production of jet and diesel fuel range hydrocarbons alkanes from hemicellulose extracts. The first step in this process was an acid-catalyzed biphasic dehydration to produce furfural. The furfural was extracted from the aqueous solution into a tetrahydrofuran (THF) phase which was then fed into an aldol condensation step. The furfural-acetone-furfural (F-Ac-F) dimer was produced in this step by reaction of furfural with acetone. The F•Ac-F dimer was then subjected to a low-temperature hydrogenation to form the hydrogenated dimer (H-FAF) at 110-130° C. and 800 psig with a 5 wt % Ru/C catalyst. Finally the H-FAF underwent hydrodeoxygenation to make jet and diesel fuel range alkanes, primarily C13 and C12 range hydrocarbons.
Ostuka and associates, U.S. Pat. No. 3,752,849, manufacture levulinic acid by ring cleavage of furfuryl alcohol. Fitzpatrick, U.S. Pat. No. 4,897,497 and U.S. Pat. No. 5,608,105, produces furfural and levulinic acid from lignocellulose and levulinic acid from carbohydrate-containing materials in high yields using two reactors in which the temperature, reaction time, and acid content are closely controlled. Van de Graaf and Lange, U.S. Pat. No. 7,265,239, use a porous strong acid ion-exchange resin as a catalyst for the conversion of furfuryl alcohol with water or an alkyl alcohol into levulinic acid or alkyl levulinate.
A variety of oxygenates can be derived from biomass carbohydrates through various techniques including acid hydrolysis, pyrolysis, fermentation, and the like. Oxygenates can be obtained via carbohydrate hydrolysis/dehydration under severe reaction conditions, i.e., temperature ≧180° C. and catalyst concentration ≧1 wt % (U.S. Pat. No. 4,897,497 and U.S. Pat. No. 5,608,105; Biometic I, 2002; Fitzpatrick, 2006; De Jong, 2010). Additionally, carbohydrate hydrolysis/dehydration can be carried out in the presence of alcohols. When alcohols are present a levulinic acid ester can also be obtained in high yields. Thus, for instance, if ethanol is introduced during the carbohydrate hydrolysis/dehydration, major products include ethyl levulinate, levulinic acid and furfurals. Depending on the reactor system used for the hydrolysis/dehydration step, furfurals and formic acid can be produced as gas phase products. When this is the case, furfurals in the gas phase can be easily hydrogenated to furfurol (Chen, 2002; Wu, 2005). Furfural can be fed to the hydrolysis/dehydration reactor to be converted to levulinic acid and/or alkyl levulinate via hydrolysis (Otsuka, 1973; Olson, 2001; van de Graaf, 2007; Lang, 2008). The levulinate and furfural products from the hydrolysis/dehydration step can be separated from water and other byproducts (tars and solid char) using a variety of separation techniques. Separation technologies including filtration, decantation, solvent extraction, and distillation, can be used to obtain purified levulinate and furfural. Levulinate, furfural, and other oxygenates are not heat stable and need to be stabilized for further processing.
Unfortunately, most biofuels cannot be used as a gasoline substitute, but must be blended with gasoline, as is the case with cellulosic ethanol. Synthesis of hydrocarbons from biological sources is limited, a more efficient and fungible source of renewable fuels is required. Thus far, there remains a need for technologies that are able to provide biomass-derived hydrocarbon fuels including gasoline, diesel and jet fuels that can be used interchangeably with a wide range of petroleum based fuels.